1. Field of the Invention
The present invention relates to a method of forming a patterned thin film and a method of fabricating a micro device in which a patterned thin film is formed through the use of an undercut mask.
2. Description of the Related Art
As disclosed in Published Unexamined Japanese Patent Application (KOKAI) No. Heisei 9-96909 (1997), for example, methods such as etching, liftoff, and a combination of etching and liftoff (hereinafter called a combination method) are available to form a patterned thin film on a base layer by using a patterned resist layer as a mask.
To perform the aforementioned methods, an undercut mask is preferably used. An undercut mask means a mask whose width at the bottom is smaller than the width at the top. Known methods of forming such a mask include those using a two-layer resist, using a resist having an image reversal function, and utilizing a micro-groove, as disclosed in Published Unexamined Japanese Patent Application (KOKAI) Nos. Heisei 2-17643 (1990), 9-96909 (1997), and 8-69111 (1996), respectively.
The method of forming an undercut mask using a two-layer resist will now be briefly described. According to this method, first, a first layer and a second layer are sequentially formed on a base layer. The second layer is made of a resist, while the first layer is made of a material that dissolves in a developing solution. Then, the second layer is exposed to specifically patterned image radiation. After the exposure, the second layer is developed and part of the first layer is dissolved in a developing solution to make the first layer smaller in width than the second layer. Thus, the patterned first and second layers make up an undercut mask.
Next, the method of forming an undercut mask using a resist having an image reversal function will be briefly described. According to this method, first, a resist layer of a resist having an image reversal function is formed on a base layer. The resist having an image reversal function is a positive resist whose portion that has become soluble in a developing solution by being exposed to radiation turns insoluble in the developing solution by being heated. Then, the resist layer is exposed to specifically patterned image radiation. Subsequently, the resist layer is heated so that the portion thereof that has become soluble in a developing solution by being exposed to the radiation turns insoluble in the developing solution. The entire surface of the resist layer is then exposed to radiation so that portions of the resist layer other than the portion having been exposed to the first radiation become soluble in the developing solution. The resist layer is then developed. Thus, the portion of the resist layer that has been exposed to the first radiation only remains. This portion serves as an undercut mask.
Next, the method of forming an undercut mask by utilizing a micro-groove will be briefly described. A micro-groove is a phenomenon in which a patterned resist layer is deformed such that the width at the bottom thereof becomes smaller than the width at the top thereof. According to this method, first, formed on a base layer is a resist layer containing an acid-generating agent for generating an acid through exposure to radiation, the acid being soluble in a developing solution. The resist layer is exposed to specifically patterned image radiation. Subsequently, the resist layer is heated so that the acid that has been generated through the exposure segregates to a region of the resist layer closer to the base layer. The resist layer is then developed. This forms a patterned resist layer in which a micro-groove has been developed, and this patterned resist layer serves as an undercut mask.
Reference is now made to FIGS. 30 to 33 to describe a method of forming a patterned thin film by means of etching through the use of an undercut mask. According to this method, as shown in FIG. 30, a film 302 to be patterned is first formed on a base layer 301. Then, as shown in FIG. 31, an undercut mask 303 is formed on the film 302 to be patterned. Then, as shown in FIG. 32, the film 302 is selectively etched by dry etching such as ion milling through the use of the mask 303, to thereby form a patterned thin film 304 in a desired shape. Then, as shown in FIG. 33, the mask 303 is peeled off.
Now, reference is made to FIGS. 34 to 38 to describe a method of forming a patterned thin film by means of the combination method through the use of an undercut mask. According to this method, as shown in FIG. 34, a first film 312 to be patterned is first formed on a base layer 311. Then, as shown in FIG. 35, an undercut mask 313 is formed on the first film 312 to be patterned. Then, as shown in FIG. 36, the first film 312 to be patterned is selectively etched by dry etching such as ion milling through the use of the mask 313, to thereby form a first patterned thin film 314 in a desired shape. Then, as shown in FIG. 37, a second film 316 to be patterned is formed by sputtering to cover the base layer 311 and the mask 313. Then, as shown in FIG. 38, the mask 313 is peeled off. A second patterned thin film 318 in a desired shape is thereby obtained. According to the combination method, the first patterned thin film 314 and the second patterned thin film 318 can be formed so as to be adjacent to each other on the base layer 311.
On the other hand, forming a patterned thin film by the etching or the combination method using an undercut mask causes the following problems.
For the case of employing etching, when the film 302 to be patterned is being selectively etched by using the mask 303 as shown in FIG. 32, a substance that forms the film 302 separates from the film 302 due to the etching. The substance then deposits on the patterned thin film 304 in the area around the bottom of the mask 303, thereby forming a deposition film 305. The deposition film 305 can cause burrs, as shown in FIG. 33, on the patterned thin film 304 after the mask 303 has been peeled off. The burrs may result in defects of the patterned thin film 304, thereby causing a reduction in the yield of products such as micro devices that include the patterned thin film 304 or an increase in the lead time in product fabrication. A micro device means a small-size device fabricated through the use of thin-film forming techniques. Examples of the micro device include semiconductor devices, thin-film magnetic heads, and sensors and actuators incorporating thin films.
On the other hand, for the case of employing the combination method, when the first film 312 to be patterned is being selectively etched by using the mask 313, a substance that forms the film 312 separates from the film 312 due to the etching. The substance then deposits on the first patterned thin film 314 in the area around the bottom of the mask 313, thereby forming a deposition film 315 as shown in FIG. 36. Further, as shown in FIG. 37, during the sputtering to form the second film 316 to be patterned, the substance for forming the film 316 also reaches the area around the bottom of the mask 313 and deposits on the first patterned thin film 314, thereby forming a deposition film 317. The deposition films 315 and 317 can cause burrs, as shown in FIG. 38, on the first patterned thin film 314 after the mask 313 has been peeled off. The burrs may result in defects of the first patterned thin film 314, thereby causing a reduction in the yield of products such as micro devices that include the first patterned thin film 314 or an increase in the lead time in product fabrication.
As described above, the conventional methods of forming a patterned thin film by means of etching or the combination method using an undercut mask may cause defects in the patterned thin film.